1. Field of the Invention
The present invention relates to a vacuum pump, such as a turbo-molecular pump, that drives to rotate a rotor using a motor.
2. Description of the Related Art
Axial flow type vacuum pumps such as turbo molecular pumps rotate rotors having a rotor blade at a high speed for evacuation. Since such vacuum pumps discharges a rarefied gas while compressing the gas, the rotors rotate in only one direction. Hereinafter, this rotational direction is called as positive rotation. Therefore, rotor rotation in the vacuum pump is normally accelerating and decelerating motions between a still region and a positive rotation region.
Conventionally, rotational speed information and magnetic pole positional information about motor rotors are obtained as information necessary for motor drive control for rotating rotors based on detection signals of rotation sensors. However, in vacuum pumps that detect targets (having gaps) provided to rotors using inductance type gap sensors, it is difficult to detect the rotational direction using only the rotation sensors.
For this reason, normally, the above problem is handled by devising a control sequence in the motor driving (particularly, at a start time when reverse rotation likely occurs) (for example, see JP 4692891 B). However, the handing method through the devising of the control sequence has a disadvantage that an actuating time period up to the positive rotation of rotors takes a long time.
Besides the system in which the inductance type gap sensors are used as rotation sensors, a rotation sensorless control is proposed in order to improve reliability and reduce a cost. Further, DC brushless motors are employed in view of energy saving, and sine wave drive is used often as a driving method. In such vacuum pumps, a magnetic pole position and a rotational speed based on a counter electromotive voltage associated with rotation of permanent magnet attached to a motor rotor.
Normally, in a middle or high speed rotational speed region, a counter electromotive voltage is calculated from a constant of an electrically equivalent circuit (a relationship between a current and a voltage is defined) approximate to an actual motor constant, and detected motor current signal and voltage signal so that the position of magnetic pole is obtained. Since the counter electromotive voltage is proportional to a rotation number, the voltage value is weak in a low-speed rotation region like a motor starting time. On the other hand, an output voltage of a three-phase inverter is generally set to a voltage value that is equivalent to or larger than the counter electromotive voltage in a rated rotation (for example, tens of volts in a case of a turbo-molecular pump.
Therefore, for example in vacuum pumps whose rated rotation number is 1000 rps, a value of a counter electromotive voltage at 1 rps just after the starting is 1/1000 of the value of a counter electromotive voltage at a rated time, and thus the value is about tens of millivolts. It is very difficult to accurately extract a weak counter electromotive voltage from a PWM output voltage where an on/off state is repeated at not less than tens of volts.